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Continuous coflow synthesis of hybrid palladium nanocrystals as catalysts for borylation reaction† Oana Pascu,a Ludovic Marciasini,b Samuel Marre,a Michel Vaultier,b Mathieu Pucheault*b and Cyril Aymonier*a The combination of highly active Pd nanocrystal (NC) types with tailored surface properties (various ligands) – e.g. organic–inorganic hybrid NCs – as catalysts opens avenues towards new synthetic
Received 25th July 2013 Accepted 2nd October 2013
pathways, implying a faster practical alternative for adjusting and screening the reaction conditions. Pd@dppf and Pd@PCy3 NCs have been successfully prepared via a continuous supercritical fluid assisted coflow route with promising results as catalysts in borylation reaction. It has been found that the
DOI: 10.1039/c3nr03858k
ligands not only influence the catalytic properties of the systems, but also contribute to Pd metal core
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characteristics (size, shape and time stability).
Introduction Hybrid nanocrystal (NC) catalysts with a metal core and organic functional shell1–3 or a metal core surrounded by a porous oxide shell4 attract high interest in semi-heterogeneous/heterogeneous catalysis among other applications due to their combined properties.2,4 On one hand, a transition-metal core not only shows increasing reactivity due to high surface-to-volume ratio, but also shows tunable characteristics, such as size, composition, morphology, stability etc. On the other hand, the organic functional shell may induce selectivity in reactions and could also inuence the catalytic properties of the core. In addition, the interaction between the inorganic core and the organic shell could be engineered to enhance the catalytic activity, selectivity and structural stability.4 The alliance between nanotechnology and catalysis can therefore lead to the production of materials with specic surface chemistry, size and shape thus achieving some peculiar requirements in catalysis, such as selectivity.3 In that respect, carbon–boron bond forming reactions (borylation) require ne catalyst tuning, as witnessed by numerous developed systems to perform Miyaura borylation with bis(pinacolato)diboron.5–7 The main product of borylation reaction, arylboronic acids and their esters, are key reagents in synthetic organic chemistry being highly stable and nontoxic.5–13 A very convenient method to prepare these arylboronate compounds is the direct borylation of aryl halides with alkoxyboranes.8,9,11 Depending on substrates (electron donating or electron withdrawing groups), ligands, bases and solvents, a
CNRS, Univ. Bordeaux, ICMCB, UPR 9048, 87 avenue du Dr Albert Schweitzer, F-33600 Pessac Cedex, France. E-mail: [email protected]
b
ISM, UMP, CNRS 5255, Universit´e Bordeaux 1, 351 course de la Lib´eration, F-33405 Talence, France. E-mail: [email protected] † Electronic supplementary 10.1039/c3nr03858k
information
(ESI)
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available.
See
DOI:
borylation reaction necessitates adjustment and screening to compensate activity losses. Therefore new synthetic routes such as semi-heterogeneous/heterogeneous borylation reaction are welcome. A combination of highly active NCs with surface property modulation (ligand type and ligand ratio with respect to the core) provides a fast and practical alternative for adjusting and screening the reaction conditions. Highly active NCs can be successfully prepared via the supercritical uid route.14 High pressure/high temperature conditions conned in a controlled environment offered by HT/HP microreactors15 provide an excellent environment for the preparation of nanomaterials with enhanced properties.16,17 At this scale not only heat and mass transfer is improved, with an increased reaction rate, but also it takes advantages of a medium that behaves both as a liquid and gas at the same time. In addition, the continuous supercritical uid coow synthesis allows the tuning of NCs' peripheral chemical functionalities by their in situ or ex situ functionalization.18 Ligands play an important role in borylation reaction.5–13,19 The catalyst efficiency depends not only on the electron donating capacity of the phosphine ligands but also on the availability of free coordination sites on the Pd active center. In the present work, two ligands displaying different electronic, steric and binding strengths to metal center properties have been chosen to study their inuence on the catalytic process and also their possible contribution to Pd NCs' physical characteristics (size, morphology, time stability). We focused on 1,10 -bis(diphenylphosphino)ferrocene (dppf) and tricyclohexyl phosphane (PCy3) ligands to prepare organic–inorganic hybrid NCs labeled as Pd@L (Pd@dppf and Pd@PCy3) (Fig. 1) due to their higher catalytic activities in borylation reaction, previously reported by our group.17 Here we report successful carbon–boron bond forming reactions using hybrid Pd NCs as catalysts, with good
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Paper gel (230–400 mesh) purchased from Merck was used for ash chromatography. Analytical TLC silica gel 60 F254 was used.
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Methodology
Fig. 1 Schematic representation of the homemade coflowing continuous reactor setup.
conversion of the substrates bearing electron donating groups, especially in para or meta substitution. Although the electron withdrawing groups have been found more difficult to borylate, the use of organic–inorganic hybrid NC catalysts opens the way for new synthetic pathways. We have observed that employing ligands with different steric and electronic properties (dppf and PCy3) can inuence the NCs size, morphology and time stability with consequences in the catalytic activity.
Experimental procedures Materials Palladium(II) bis(hexauoroacetylacetonate) (Pd(hfa)2, 99%) and 1,10 -bis(diphenylphosphino)ferrocene (dppf) were purchased from Strem Chemicals and used without further purication. Tri(cyclohexyl)phosphane (PCy3), 20 ,60 -dimethoxybiphenyl, bis(pinacolato)diboron, triethylamine, 1,3,5-mesitylene, potassium acetate and potassium carbonate were purchased from Sigma-Aldrich and used without further purication. 1,3-Bis(2,3,5-trimethylphenyl)imidazolium tetrauoroborate was prepared as described in the literature.20 Pinacol was purchased from Sigma-Aldrich and distilled before use. Aryl bromide was purchased from Fischer Scientic and used without further purication. Di(isopropyl)aminoborane was prepared as described in the literature.21 A pressurized tank of a CO2–H2 mixture (80 : 20 molar) was used as received from Air Liquid. All catalytic reactions were carried out in an inert atmosphere (Ar). All chemicals and nanoparticle solutions were stored under argon. Toluene and methanol were distilled over CaH2. Isolated yields refer to chromatographically and spectroscopically (NMR and GC-MS) homogeneous materials. Silica
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Continuous synthesis of Pd@L nanocrystals. The homemade continuous reactor setup used to synthesize the Pd@L NCs was similar to the one previously reported by our group.15 Briey, the continuous synthesis (Fig. 1) was carried out in a coaxial owing reactor: (1) a fused silica capillary (inner diameter (I.D.): 250 mm, outer diameter (O.D.): 360 mm, length: 20 cm) inserted in (2) an external stainless steel tubing (I.D.: 750 mm, O.D.: 1587.5 mm, length: 1.3 m). The length of the external tubes from the conuence region to the outlet was 1 m. The heating of the reactor (100 C) was provided by a silicone oil bath, magnetically stirred to ensure a homogeneous temperature, while the pressure was controlled with a back-pressure regulator downstream (25 MPa). The metal precursor (Pd(hfac)2) with 9.1 102 M concentration in toluene solution (SP) was injected with a high-pressure pump (HPP) at 25 MPa, room temperature (RT) at a ow rate of 35 mL min1 (Q2) into the T-type mixing point where it met the stream of a CO2 and H2 mixture (the molar ratio of CO2 to H2 being 80 to 20) at 25 MPa, RT and 70 mL min1 ow rate (Q3). A second solution (SL) containing the organic ligand (dppf/PCy3) in toluene (5.36 103 M for bidentate–dppf and 1.08 102 M monodentate– PCy3 ligands) was injected externally with a third high pressure pump at a ow rate of 1395 mL min1 (Q1) into the tubular reactor. The ligands-to-metal precursor molar ratio was calculated to have the same phosphorus quantity per precursor molecule, e.g. 4 atoms of P to 1 of Pd. In this regard, we have employed 2.3 dppf and 4.7 PCy3 molecules per Pd precursor molecule, respectively. The overall residence time (Rt) was set to be 17.6 s. The outlet, containing the Pd@dppf or Pd@PCy3 NCs dispersed in toluene, was collected and centrifuged at 9000 rpm for 30 min. The resulting supernatant was used directly for catalytic reactions. For NC characterization, further separation and cleaning steps were performed; the Pd@L toluene dispersion was rst concentrated by removing the toluene under reduced pressure, ethanol was then added and the formed black precipitate was separated by centrifugation (10 000 rpm for 30 min). Finally the precipitate was dried overnight and re-dispersed in toluene. A few drops from the very concentrated dispersion on a silicon monocrystal support were used for XRD measurements, while TEM was performed using a very dilute dispersion. Borylation reaction procedure using di(isopropyl)aminoborane. A septum-capped vial, equipped with a magnetic stirring bar, was charged with Pd@L NC toluene dispersion (5 mL, 1 mol% Pd). The vial was closed and concentrated under reduced pressure (ca. 1 mL). Aryl bromide (1.0 mmol), di(isopropyl)aminoborane (331 mL, 1.5 mmol; 1.5 eq.) and triethylamine (404 mL; 3.0 mmol; 3.0 eq.) were added and the reaction mixture was evacuated under vacuum, placed in an argon atmosphere and stirred at 100 C for 1 h (preheated oil bath). The mixture was cooled at 0 C and methanol (2 mL) was added.
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The mixture was then allowed to warm to room temperature and stirred for 1 h. The mixture was concentrated under vacuum, pinacol (153 mg, 1.3 mmol, 1.3 eq.) in Et2O (2 mL) was added to the residue and the reaction mixture was stirred for 4 h. Et2O (10 mL) was added, and the reaction mixture was ltered over a celite pad. The resulting organic layer was washed with an aqueous solution of CuCl2 (3–5 mL, 50 g L1). The organic layer was dried over Na2SO4 and concentrated under reduced pressure to afford analytically pure boronate. Purication with column chromatography was performed if required to attain the analytical purity. Characterization Morphological and structural characterization were performed by transmission electron microscopy (TEM, Jeol 2200FS) and Xray powder diffraction (PANalytical X'Pert Pro with Cu lKa radiation). The TEM samples were prepared by depositing a drop of diluted toluene NCs dispersion onto a carbon grid and letting the solvent to evaporate. The mean diameter and polydispersity were determined by tting a particle size histogram (measured from TEM images by using the imageJ soware) to a Gaussian distribution. GC-MS analysis was performed with a HP 6890 series GC-system equipped with a J&W Scientic DB-1701 capillary column, and a HP 5973 mass selective detector (EI) using the following method: the temperature was held at 70 C for 1 min, then the temperature was increased till 230 C with a heating rate of 20 C min1 and kept for 6 min at 230 C.
these conditions, the molar fraction of toluene in the reaction media was 0.9. By using continuous coaxial ow capillary microsystems, metal/metal oxide nanocrystals with enhanced properties can be prepared.15,16,22 The advantage of this setup is the ability to conduct separately the nucleation/growth and functionalization (with organic ligands) steps. Depending on the average velocities of the external to the inner ow (RH) two main regimes could appear, the ow-focusing regime with RH > 1 and the owspreading regime with RH < 1,22 inuencing the interaction between nucleation/growth and NCs functionalization processes.15,22,23 In the above described setup, RH was 1.66 inducing a ow-focusing regime. Structure and morphology Powder X-ray diffraction patterns of Pd@dppf and Pd@PCy3 are presented in Fig. 2. The reections in the patterns can be indexed with the face-centered cubic phase of palladium (ICDD PDF 046-1043), but are slightly shied. The calculated lattice parameters for (111) reection in the case of Pd@dppf and ˚ and 3.909 A, ˚ respectively. All these values Pd@PCy3 were 3.94 A have been found to be somewhat larger than the value of bulk ˚ The expansion of the lattice unit, already Pd (a ¼ 3.89 A). observed by others,24–26 could be attributed to the formation of a second phase, insertion of C24,27 and/or H26 atoms into the octahedral sites of f.c.c. Pd. Although cleaning and separation steps were performed for both Pd@L systems, traces of the organic ligand are observed in XRD diffraction patterns of
Results and discussions We herein describe the synthesis of hybrid palladium NCs using a continuous coaxial ow process. The metal precursor, bis(hexauoroacetylacetonate) palladium(II), is reduced with hydrogen in a toluene–CO2 + H2 mixture as reaction media and in the presence of an organic ligand with a double role: stabilizing and modifying the stereoelectronic properties of NCs.15 We envisioned with this continuous setup the as-obtained solution to be directly used for catalysis (borylation reaction) and in this regard, the toluene has been chosen as the reaction medium purposefully. Supercritical CO2 aims at enhancing the H2 solubility in toluene, therefore overcoming the mass-transfer limitation for the Pd precursor reduction. The experimental parameters used in this work are presented in Table 1. Under Table 1 Experimental conditions used for the continuous synthesis of hybrid Pd@L NCs
Pd@L
Pd@dppf
Prec [prec] (M) [L] (M) QP (ml min1) QL (ml min1) QCO2+H2 (ml min1) T ( C) p (MPa) Rt (s)
Pd(hfac)2 9.1 102 5.36 103 35 1395 70 100 25 17.6
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Pd@PCy3
1.08 103 35 1395 70 Fig. 2 Powder XRD diffraction patterns of both Pd@dppf and Pd@PCy3 systems in comparison with the bulk Pd.
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Pd@PCy3 (Fig. 2-marked with the star symbol). The XRD of both pure ligands are shown in the ESI (Fig. S1†). The TEM images of the as prepared hybrid inorganic– organic Pd NCs show differences in size and morphology for the two systems, Pd@dppf and Pd@PCy3. Pd@dppf NCs (Fig. 3) present a main population of small NCs, around 2.4 nm, monodispersed, almost spherical, with a narrow size distribution (Fig. 3b), very good crystallinity, and visible lattice fringes (Fig. 3c). In contrast, the Pd NCs capped with PCy3 molecules are more agglomerated in a cloud of the organic ligand (Fig. 4a), having at the same time a more irregular shape and being a bit larger (Fig. 4b). For this system it was very difficult to determine the NCs size and make the Gaussian distribution. In our systems, the ligands used are monophosphanes (PCy3) or bisphosphanes (dppf). In the present work, we observed that the ligand type (their different steric, and electronic properties and molecule sizes) has an effect on the nal NC characteristics (size, morphology and time stability). This situation might be attributed to the ligands. Due to its pair of electrons, P is able not only to coordinate the surface of Pd NCs but also could play the role of a reducing agent for Pd(II) to Pd(0), therefore inuencing both the interaction between organic ligand–inorganic metal core and the system catalytic properties.
Fig. 4 Toluene dispersion of fresh (a and b) of Pd@PCy3 NCs. TEM images of agglomerated NCs embedded in a cloud of the organic ligand (a); a higher magnification image of NCs with undefined shapes (b).
Fig. 3 Fresh toluene dispersion of Pd@dppf NCs. TEM images of dispersed NCs (a), Gaussian distribution of presented NCs (b) and HRTEM of a few NCs (c).
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PCy3 ligand bearing electron-rich phosphorus complexes are stronger reducing agents than dppf. As a consequence, in the system containing the metal precursor and PCy3, additional reduction of Pd ions from the metal precursor by the PCy3 ligand probably occurs resulting in bigger Pd NCs. By reducing the Pd(II), the ligand will be oxidized (phosphine oxide), losing its ability to coordinate the Pd NC surface through the lone pair electrons, leaving the NC surface less covered. In addition, the repulsion between electron-rich PCy3 molecules might impede the presence of too many ligand molecules around the Pd core. This scenario could explain the larger growth to reach the thermodynamic equilibrium. In contrast, in dppf molecules, the electron pair of P, due to the resonance effect with the attached phenyl groups (pi electrons of the ring), could be less available for reducing but still able to coordinate the Pd NCs. In addition, being a bulkier ligand, dppf may form a dense shell around the metal core, shielding the core from other coming Pd nuclei, thus stopping the NCs growth. As a result, the formed nanocrystals are very small, but better dispersed, as conrmed by TEM images (Fig. 3a and 5a). Over time, sorption/desorption of the ligand may take place and due to the higher reactivity of the small Pd NCs, their further growth through the Ostwald
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Nanoscale Table 2 Selected synthesis of boronic esters using two different palladium NCs systems (a purified by flash chromatography)
Yielda [%]
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Entry
Fig. 5 TEM images showing the time stability of Pd@dppf toluene dispersion: fresh material (a), and after two month toluene storage (b). Higher magnification of (b) image, presenting big nanoparticles surrounded by small ones (c). TEM images of Pd@PCy3 NCs after a few month toluene storage (d).
ripening mechanism may occur. This was conrmed by TEM analysis of the Pd@dppf system aer two months of storage in toluene dispersion (Fig. 5b). At the rst glance very big particles can be seen but looking at a higher magnication (Fig. 5c), small nanoparticles surrounding the big ones have been observed, being in agreement with the proposed mechanism. The bigger NPs have no round shape but are more of (truncated) tetrahedron28 or triangular shape.29 It is known30–32 that starting from single-crystal seeds, a variety of shapes can be obtained only by altering the surface free energy of different facets with a specic capping agent. The tetrahedral shape is covered only by {111} facets which have the lowest free surface energy of the three facets {111} < {100} < {110}.33 According to this, the formation of the tetrahedral shape might be energetically favored, and this could explain their presence in toluene dispersion aer 2 month storage. Their catalytic activity will be discussed in the next section. The colloidal behaviour of Pd@PCy3 is very different from Pd@dppf. From the beginning, in fresh toluene dispersion, the NCs are larger in size (Fig. 5d). By analysing the toluene dispersion aer 2 months of storage (Fig. 5d) not much difference can be observed. It seems that the formed NCs are stable in time, probably because they have already reached the thermodynamic equilibrium. This is in agreement also with their unchanged catalytic activity (see the discussion below).
Product
Pd@dppf
Pd@Pcy3
1
76
100
2
94
88
3
98
99
4
94
91
5
99
83
6
91
100
7
65a
58a
8
37a
45a
9
55a
51a
10
62a
54a
11
70a
74a
12
33a
28a
13
11a
41a
14
53a
56a
15
71a
67a
The reaction was performed at 100 C with triethylamine as the base and 1 mol% of freshly synthesized Pd NCs dispersed in toluene. All the reactions went to completion and the aryl aminoborane intermediate was then converted to the corresponding pinacol ester by a methanolysis/transesteriaction sequence. In some cases, a simple ltration over a celite pad is enough to afford analytically pure boronate. a
Borylation reaction As already mentioned, the goal of the work was the direct use of the outlet solution (containing hybrid Pd NCs) from the continuous setup as a semi-heterogeneous catalyst for
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Fig. 6
One pot Suzuki borylation followed by Suzuki–Miyaura cross coupling.
borylation reaction. The ligands used (dppf, PCy3) are already known for their role in homogeneous catalysis for the same borylation reaction. By employing these ligands together with metal Pd NCs, new pathways in the catalysis eld can be opened. In this section we present the further investigations on borylation reactions catalysed by organic–inorganic hybrid Pd NCs, previously reported by our group.17 Pd@dppf and Pd@PCy3 have been chosen as catalysts due to their high activity in the borylation reaction of 4-bromoanisole with di isopropylaminoborane instead of the traditional bis(pinacolato) diboron or pinacolborane.34 For some substrates bearing no substitution (Table 2, entry 1) or electron donating groups (Me, OMe and NMe2, Table 2, entries 2–6), the reactions proceed cleanly and a simple ltration over a silica pad is enough to afford pure boronate. For the two catalyst types (Pd@PCy3 and Pd@dppf NCs) the yields are very similar except for the non-substituted arene ring (entry 1) where the Pd@PCy3 NCs produced a quantitative yield in boronate. These results are in agreement with the expected ones based on our previous work. In the case of polysubstituted substrates and other arene type substrates, moderate yields are obtained for both the systems. Note that in all these series, column chromatography was necessary because the reduction product (e.g. substitution of the bromide by a hydrogen) cannot be removed under vacuum (except for entries 12 and 13, Table 2) and in some cases, the reaction does not go to completion (entries 8, 12 and 13). The 2-OMe and 2-Me substitution (entries 12–13, Table 2) was found to be difficult to borylate due to steric hindrance of the reactive center. Compared with the acceptable yield obtained in homogeneous catalysis for the substrates bearing electron-withdrawing groups, the rst results, using organic–inorganic hybrid Pd NCs, obtained in the eld open an avenue for further investigation. Nonetheless, those NCs can be used to perform a one pot borylation–cross coupling sequence (Fig. 6),17,21 using the same palladium source for both reactions. In terms of time stability and catalytic efficiency of the hybrid Pd NCs, we performed borylation reaction with a solution aer a few months of storage in toluene. In the case of the Pd@dppf system the reaction yield decreased from 98% (fresh solution) to 50% (Table 2, entry 3), not surprising for a system containing the biggest NPs (Fig. 5c). Similar catalytic activity (yield 98%, Table 2, entry 3) for the Pd@PCy3, fresh and stored toluene dispersion was found, as expected for a thermodynamically stable system.
Conclusions In summary, a combination of highly active Pd NCs with surface property modulation (different ligands) leading to
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Paper organic–inorganic hybrid NC types, prepared via the continuous supercritical uid assisted coow route, seems to be promising catalysts in borylation reactions. Indeed, regardless of the meta or para substitution, bromides were converted almost quantitatively to the corresponding boronates, without the need for column chromatography. Sterically demanding aryl bromides were found to be more difficult to transform cleanly because the reduction was the favored reaction mechanism. Although the employed ligands in homogeneous catalysis give good results for the borylation reaction, even for substrates bearing electron withdrawing groups, in a colloidal system, hybrid type, the behaviour is different, conrming again the strong reactivity difference between homogeneous and semi-heterogeneous catalysts. Even so, the use of hybrid inorganic–organic NCs as catalysts opens the way for new synthetic pathways, enabling the possibility of catalyst surface modulation. This implies a faster practical alternative for adjusting and screening the reaction conditions.
Acknowledgements Acknowledgments are given to the R´ egion Aquitaine, GIS AMA and the ANR NANOCAUSYS no. 12-CDII-0010 for funding.
Notes and references 1 D. Astruc, F. Lu and J. Ruiz Aranzaes, Angew. Chem., Int. Ed., 2005, 44, 7852. 2 T. Mitsudome, Y. Takahasi, S. Ichikawa, T. Tizugaki, K. Jitsukawa and K. Kaneda, Angew. Chem., Int. Ed., 2013, 52, 1481. 3 F. Zaera, Chem. Soc. Rev., 2013, 42, 2746. 4 Q. Zhang, I. Lee, J. B. Joo, F. Zaera and Y. Yin, Acc. Chem. Res., 2013, 46, 1816. 5 T. Ishiyama, M. Murata and N. Miyaura, J. Org. Chem., 1995, 60, 7508. 6 W. K. Chow, O. Y. Yuen, P. Y. Choy, C. M. So, C. P. Lau, W. T. Wong and F. Y. Kwong, RSC Adv., 2013, 3, 12518. 7 A. Bej, D. Srimani and A. Sarkar, Green Chem., 2012, 14, 661. 8 M. Murata, T. Oyama, S. Watanabe and Y. Masuda, J. Org. Chem., 2000, 65, 164. 9 O. Baudoin, D. Guenard and F. Gueritte, J. Org. Chem., 2000, 65, 9268. 10 G. A. Molander, S. L. J. Trice, S. M. Kennedy, S. D. Dreher and M. T. Tudge, J. Am. Chem. Soc., 2012, 134, 11667. 11 W. K. Chow, O. Y. Yuen, C. M. So, W. T. Wong and F. Y. Kwong, J. Org. Chem., 2012, 77, 3543. 12 A. Bej, D. Srimani and A. Sarkar, Green Chem., 2012, 14, 661. 13 J. Takagi and T. Yamakawa, Tetrahedron Lett., 2013, 54, 166. 14 F. Cansell and C. Aymonier, J. Supercrit. Fluids, 2009, 47, 508. 15 S. Marre, Y. Roig and C. Aymonier, J. Supercrit. Fluids, 2012, 66, 251. 16 Y. Roig, S. Marre, T. Cardinal and C. Aymonier, Angew. Chem., Int. Ed., 2011, 50, 12071. 17 Th. Gendrineau, S. Marre, M. Vaultier, M. Pucheault and C. Aymonier, Angew. Chem., Int. Ed., 2012, 51, 8525–8528.
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Continuous coflow synthesis of hybrid palladium nanocrystals as catalysts for borylation reaction† Oana Pascu,a Ludovic Marciasini,b Samuel Marre,a Michel Vaultier,b Mathieu Pucheault*b and Cyril Aymonier*a The combination of highly active Pd nanocrystal (NC) types with tailored surface properties (various ligands) – e.g. organic–inorganic hybrid NCs – as catalysts opens avenues towards new synthetic
Received 25th July 2013 Accepted 2nd October 2013
pathways, implying a faster practical alternative for adjusting and screening the reaction conditions. Pd@dppf and Pd@PCy3 NCs have been successfully prepared via a continuous supercritical fluid assisted coflow route with promising results as catalysts in borylation reaction. It has been found that the
DOI: 10.1039/c3nr03858k
ligands not only influence the catalytic properties of the systems, but also contribute to Pd metal core
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characteristics (size, shape and time stability).
Introduction Hybrid nanocrystal (NC) catalysts with a metal core and organic functional shell1–3 or a metal core surrounded by a porous oxide shell4 attract high interest in semi-heterogeneous/heterogeneous catalysis among other applications due to their combined properties.2,4 On one hand, a transition-metal core not only shows increasing reactivity due to high surface-to-volume ratio, but also shows tunable characteristics, such as size, composition, morphology, stability etc. On the other hand, the organic functional shell may induce selectivity in reactions and could also inuence the catalytic properties of the core. In addition, the interaction between the inorganic core and the organic shell could be engineered to enhance the catalytic activity, selectivity and structural stability.4 The alliance between nanotechnology and catalysis can therefore lead to the production of materials with specic surface chemistry, size and shape thus achieving some peculiar requirements in catalysis, such as selectivity.3 In that respect, carbon–boron bond forming reactions (borylation) require ne catalyst tuning, as witnessed by numerous developed systems to perform Miyaura borylation with bis(pinacolato)diboron.5–7 The main product of borylation reaction, arylboronic acids and their esters, are key reagents in synthetic organic chemistry being highly stable and nontoxic.5–13 A very convenient method to prepare these arylboronate compounds is the direct borylation of aryl halides with alkoxyboranes.8,9,11 Depending on substrates (electron donating or electron withdrawing groups), ligands, bases and solvents, a
CNRS, Univ. Bordeaux, ICMCB, UPR 9048, 87 avenue du Dr Albert Schweitzer, F-33600 Pessac Cedex, France. E-mail: [email protected]
b
ISM, UMP, CNRS 5255, Universit´e Bordeaux 1, 351 course de la Lib´eration, F-33405 Talence, France. E-mail: [email protected] † Electronic supplementary 10.1039/c3nr03858k
information
(ESI)
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available.
See
DOI:
borylation reaction necessitates adjustment and screening to compensate activity losses. Therefore new synthetic routes such as semi-heterogeneous/heterogeneous borylation reaction are welcome. A combination of highly active NCs with surface property modulation (ligand type and ligand ratio with respect to the core) provides a fast and practical alternative for adjusting and screening the reaction conditions. Highly active NCs can be successfully prepared via the supercritical uid route.14 High pressure/high temperature conditions conned in a controlled environment offered by HT/HP microreactors15 provide an excellent environment for the preparation of nanomaterials with enhanced properties.16,17 At this scale not only heat and mass transfer is improved, with an increased reaction rate, but also it takes advantages of a medium that behaves both as a liquid and gas at the same time. In addition, the continuous supercritical uid coow synthesis allows the tuning of NCs' peripheral chemical functionalities by their in situ or ex situ functionalization.18 Ligands play an important role in borylation reaction.5–13,19 The catalyst efficiency depends not only on the electron donating capacity of the phosphine ligands but also on the availability of free coordination sites on the Pd active center. In the present work, two ligands displaying different electronic, steric and binding strengths to metal center properties have been chosen to study their inuence on the catalytic process and also their possible contribution to Pd NCs' physical characteristics (size, morphology, time stability). We focused on 1,10 -bis(diphenylphosphino)ferrocene (dppf) and tricyclohexyl phosphane (PCy3) ligands to prepare organic–inorganic hybrid NCs labeled as Pd@L (Pd@dppf and Pd@PCy3) (Fig. 1) due to their higher catalytic activities in borylation reaction, previously reported by our group.17 Here we report successful carbon–boron bond forming reactions using hybrid Pd NCs as catalysts, with good
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Paper gel (230–400 mesh) purchased from Merck was used for ash chromatography. Analytical TLC silica gel 60 F254 was used.
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Methodology
Fig. 1 Schematic representation of the homemade coflowing continuous reactor setup.
conversion of the substrates bearing electron donating groups, especially in para or meta substitution. Although the electron withdrawing groups have been found more difficult to borylate, the use of organic–inorganic hybrid NC catalysts opens the way for new synthetic pathways. We have observed that employing ligands with different steric and electronic properties (dppf and PCy3) can inuence the NCs size, morphology and time stability with consequences in the catalytic activity.
Experimental procedures Materials Palladium(II) bis(hexauoroacetylacetonate) (Pd(hfa)2, 99%) and 1,10 -bis(diphenylphosphino)ferrocene (dppf) were purchased from Strem Chemicals and used without further purication. Tri(cyclohexyl)phosphane (PCy3), 20 ,60 -dimethoxybiphenyl, bis(pinacolato)diboron, triethylamine, 1,3,5-mesitylene, potassium acetate and potassium carbonate were purchased from Sigma-Aldrich and used without further purication. 1,3-Bis(2,3,5-trimethylphenyl)imidazolium tetrauoroborate was prepared as described in the literature.20 Pinacol was purchased from Sigma-Aldrich and distilled before use. Aryl bromide was purchased from Fischer Scientic and used without further purication. Di(isopropyl)aminoborane was prepared as described in the literature.21 A pressurized tank of a CO2–H2 mixture (80 : 20 molar) was used as received from Air Liquid. All catalytic reactions were carried out in an inert atmosphere (Ar). All chemicals and nanoparticle solutions were stored under argon. Toluene and methanol were distilled over CaH2. Isolated yields refer to chromatographically and spectroscopically (NMR and GC-MS) homogeneous materials. Silica
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Continuous synthesis of Pd@L nanocrystals. The homemade continuous reactor setup used to synthesize the Pd@L NCs was similar to the one previously reported by our group.15 Briey, the continuous synthesis (Fig. 1) was carried out in a coaxial owing reactor: (1) a fused silica capillary (inner diameter (I.D.): 250 mm, outer diameter (O.D.): 360 mm, length: 20 cm) inserted in (2) an external stainless steel tubing (I.D.: 750 mm, O.D.: 1587.5 mm, length: 1.3 m). The length of the external tubes from the conuence region to the outlet was 1 m. The heating of the reactor (100 C) was provided by a silicone oil bath, magnetically stirred to ensure a homogeneous temperature, while the pressure was controlled with a back-pressure regulator downstream (25 MPa). The metal precursor (Pd(hfac)2) with 9.1 102 M concentration in toluene solution (SP) was injected with a high-pressure pump (HPP) at 25 MPa, room temperature (RT) at a ow rate of 35 mL min1 (Q2) into the T-type mixing point where it met the stream of a CO2 and H2 mixture (the molar ratio of CO2 to H2 being 80 to 20) at 25 MPa, RT and 70 mL min1 ow rate (Q3). A second solution (SL) containing the organic ligand (dppf/PCy3) in toluene (5.36 103 M for bidentate–dppf and 1.08 102 M monodentate– PCy3 ligands) was injected externally with a third high pressure pump at a ow rate of 1395 mL min1 (Q1) into the tubular reactor. The ligands-to-metal precursor molar ratio was calculated to have the same phosphorus quantity per precursor molecule, e.g. 4 atoms of P to 1 of Pd. In this regard, we have employed 2.3 dppf and 4.7 PCy3 molecules per Pd precursor molecule, respectively. The overall residence time (Rt) was set to be 17.6 s. The outlet, containing the Pd@dppf or Pd@PCy3 NCs dispersed in toluene, was collected and centrifuged at 9000 rpm for 30 min. The resulting supernatant was used directly for catalytic reactions. For NC characterization, further separation and cleaning steps were performed; the Pd@L toluene dispersion was rst concentrated by removing the toluene under reduced pressure, ethanol was then added and the formed black precipitate was separated by centrifugation (10 000 rpm for 30 min). Finally the precipitate was dried overnight and re-dispersed in toluene. A few drops from the very concentrated dispersion on a silicon monocrystal support were used for XRD measurements, while TEM was performed using a very dilute dispersion. Borylation reaction procedure using di(isopropyl)aminoborane. A septum-capped vial, equipped with a magnetic stirring bar, was charged with Pd@L NC toluene dispersion (5 mL, 1 mol% Pd). The vial was closed and concentrated under reduced pressure (ca. 1 mL). Aryl bromide (1.0 mmol), di(isopropyl)aminoborane (331 mL, 1.5 mmol; 1.5 eq.) and triethylamine (404 mL; 3.0 mmol; 3.0 eq.) were added and the reaction mixture was evacuated under vacuum, placed in an argon atmosphere and stirred at 100 C for 1 h (preheated oil bath). The mixture was cooled at 0 C and methanol (2 mL) was added.
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The mixture was then allowed to warm to room temperature and stirred for 1 h. The mixture was concentrated under vacuum, pinacol (153 mg, 1.3 mmol, 1.3 eq.) in Et2O (2 mL) was added to the residue and the reaction mixture was stirred for 4 h. Et2O (10 mL) was added, and the reaction mixture was ltered over a celite pad. The resulting organic layer was washed with an aqueous solution of CuCl2 (3–5 mL, 50 g L1). The organic layer was dried over Na2SO4 and concentrated under reduced pressure to afford analytically pure boronate. Purication with column chromatography was performed if required to attain the analytical purity. Characterization Morphological and structural characterization were performed by transmission electron microscopy (TEM, Jeol 2200FS) and Xray powder diffraction (PANalytical X'Pert Pro with Cu lKa radiation). The TEM samples were prepared by depositing a drop of diluted toluene NCs dispersion onto a carbon grid and letting the solvent to evaporate. The mean diameter and polydispersity were determined by tting a particle size histogram (measured from TEM images by using the imageJ soware) to a Gaussian distribution. GC-MS analysis was performed with a HP 6890 series GC-system equipped with a J&W Scientic DB-1701 capillary column, and a HP 5973 mass selective detector (EI) using the following method: the temperature was held at 70 C for 1 min, then the temperature was increased till 230 C with a heating rate of 20 C min1 and kept for 6 min at 230 C.
these conditions, the molar fraction of toluene in the reaction media was 0.9. By using continuous coaxial ow capillary microsystems, metal/metal oxide nanocrystals with enhanced properties can be prepared.15,16,22 The advantage of this setup is the ability to conduct separately the nucleation/growth and functionalization (with organic ligands) steps. Depending on the average velocities of the external to the inner ow (RH) two main regimes could appear, the ow-focusing regime with RH > 1 and the owspreading regime with RH < 1,22 inuencing the interaction between nucleation/growth and NCs functionalization processes.15,22,23 In the above described setup, RH was 1.66 inducing a ow-focusing regime. Structure and morphology Powder X-ray diffraction patterns of Pd@dppf and Pd@PCy3 are presented in Fig. 2. The reections in the patterns can be indexed with the face-centered cubic phase of palladium (ICDD PDF 046-1043), but are slightly shied. The calculated lattice parameters for (111) reection in the case of Pd@dppf and ˚ and 3.909 A, ˚ respectively. All these values Pd@PCy3 were 3.94 A have been found to be somewhat larger than the value of bulk ˚ The expansion of the lattice unit, already Pd (a ¼ 3.89 A). observed by others,24–26 could be attributed to the formation of a second phase, insertion of C24,27 and/or H26 atoms into the octahedral sites of f.c.c. Pd. Although cleaning and separation steps were performed for both Pd@L systems, traces of the organic ligand are observed in XRD diffraction patterns of
Results and discussions We herein describe the synthesis of hybrid palladium NCs using a continuous coaxial ow process. The metal precursor, bis(hexauoroacetylacetonate) palladium(II), is reduced with hydrogen in a toluene–CO2 + H2 mixture as reaction media and in the presence of an organic ligand with a double role: stabilizing and modifying the stereoelectronic properties of NCs.15 We envisioned with this continuous setup the as-obtained solution to be directly used for catalysis (borylation reaction) and in this regard, the toluene has been chosen as the reaction medium purposefully. Supercritical CO2 aims at enhancing the H2 solubility in toluene, therefore overcoming the mass-transfer limitation for the Pd precursor reduction. The experimental parameters used in this work are presented in Table 1. Under Table 1 Experimental conditions used for the continuous synthesis of hybrid Pd@L NCs
Pd@L
Pd@dppf
Prec [prec] (M) [L] (M) QP (ml min1) QL (ml min1) QCO2+H2 (ml min1) T ( C) p (MPa) Rt (s)
Pd(hfac)2 9.1 102 5.36 103 35 1395 70 100 25 17.6
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Pd@PCy3
1.08 103 35 1395 70 Fig. 2 Powder XRD diffraction patterns of both Pd@dppf and Pd@PCy3 systems in comparison with the bulk Pd.
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Pd@PCy3 (Fig. 2-marked with the star symbol). The XRD of both pure ligands are shown in the ESI (Fig. S1†). The TEM images of the as prepared hybrid inorganic– organic Pd NCs show differences in size and morphology for the two systems, Pd@dppf and Pd@PCy3. Pd@dppf NCs (Fig. 3) present a main population of small NCs, around 2.4 nm, monodispersed, almost spherical, with a narrow size distribution (Fig. 3b), very good crystallinity, and visible lattice fringes (Fig. 3c). In contrast, the Pd NCs capped with PCy3 molecules are more agglomerated in a cloud of the organic ligand (Fig. 4a), having at the same time a more irregular shape and being a bit larger (Fig. 4b). For this system it was very difficult to determine the NCs size and make the Gaussian distribution. In our systems, the ligands used are monophosphanes (PCy3) or bisphosphanes (dppf). In the present work, we observed that the ligand type (their different steric, and electronic properties and molecule sizes) has an effect on the nal NC characteristics (size, morphology and time stability). This situation might be attributed to the ligands. Due to its pair of electrons, P is able not only to coordinate the surface of Pd NCs but also could play the role of a reducing agent for Pd(II) to Pd(0), therefore inuencing both the interaction between organic ligand–inorganic metal core and the system catalytic properties.
Fig. 4 Toluene dispersion of fresh (a and b) of Pd@PCy3 NCs. TEM images of agglomerated NCs embedded in a cloud of the organic ligand (a); a higher magnification image of NCs with undefined shapes (b).
Fig. 3 Fresh toluene dispersion of Pd@dppf NCs. TEM images of dispersed NCs (a), Gaussian distribution of presented NCs (b) and HRTEM of a few NCs (c).
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PCy3 ligand bearing electron-rich phosphorus complexes are stronger reducing agents than dppf. As a consequence, in the system containing the metal precursor and PCy3, additional reduction of Pd ions from the metal precursor by the PCy3 ligand probably occurs resulting in bigger Pd NCs. By reducing the Pd(II), the ligand will be oxidized (phosphine oxide), losing its ability to coordinate the Pd NC surface through the lone pair electrons, leaving the NC surface less covered. In addition, the repulsion between electron-rich PCy3 molecules might impede the presence of too many ligand molecules around the Pd core. This scenario could explain the larger growth to reach the thermodynamic equilibrium. In contrast, in dppf molecules, the electron pair of P, due to the resonance effect with the attached phenyl groups (pi electrons of the ring), could be less available for reducing but still able to coordinate the Pd NCs. In addition, being a bulkier ligand, dppf may form a dense shell around the metal core, shielding the core from other coming Pd nuclei, thus stopping the NCs growth. As a result, the formed nanocrystals are very small, but better dispersed, as conrmed by TEM images (Fig. 3a and 5a). Over time, sorption/desorption of the ligand may take place and due to the higher reactivity of the small Pd NCs, their further growth through the Ostwald
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Nanoscale Table 2 Selected synthesis of boronic esters using two different palladium NCs systems (a purified by flash chromatography)
Yielda [%]
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Entry
Fig. 5 TEM images showing the time stability of Pd@dppf toluene dispersion: fresh material (a), and after two month toluene storage (b). Higher magnification of (b) image, presenting big nanoparticles surrounded by small ones (c). TEM images of Pd@PCy3 NCs after a few month toluene storage (d).
ripening mechanism may occur. This was conrmed by TEM analysis of the Pd@dppf system aer two months of storage in toluene dispersion (Fig. 5b). At the rst glance very big particles can be seen but looking at a higher magnication (Fig. 5c), small nanoparticles surrounding the big ones have been observed, being in agreement with the proposed mechanism. The bigger NPs have no round shape but are more of (truncated) tetrahedron28 or triangular shape.29 It is known30–32 that starting from single-crystal seeds, a variety of shapes can be obtained only by altering the surface free energy of different facets with a specic capping agent. The tetrahedral shape is covered only by {111} facets which have the lowest free surface energy of the three facets {111} < {100} < {110}.33 According to this, the formation of the tetrahedral shape might be energetically favored, and this could explain their presence in toluene dispersion aer 2 month storage. Their catalytic activity will be discussed in the next section. The colloidal behaviour of Pd@PCy3 is very different from Pd@dppf. From the beginning, in fresh toluene dispersion, the NCs are larger in size (Fig. 5d). By analysing the toluene dispersion aer 2 months of storage (Fig. 5d) not much difference can be observed. It seems that the formed NCs are stable in time, probably because they have already reached the thermodynamic equilibrium. This is in agreement also with their unchanged catalytic activity (see the discussion below).
Product
Pd@dppf
Pd@Pcy3
1
76
100
2
94
88
3
98
99
4
94
91
5
99
83
6
91
100
7
65a
58a
8
37a
45a
9
55a
51a
10
62a
54a
11
70a
74a
12
33a
28a
13
11a
41a
14
53a
56a
15
71a
67a
The reaction was performed at 100 C with triethylamine as the base and 1 mol% of freshly synthesized Pd NCs dispersed in toluene. All the reactions went to completion and the aryl aminoborane intermediate was then converted to the corresponding pinacol ester by a methanolysis/transesteriaction sequence. In some cases, a simple ltration over a celite pad is enough to afford analytically pure boronate. a
Borylation reaction As already mentioned, the goal of the work was the direct use of the outlet solution (containing hybrid Pd NCs) from the continuous setup as a semi-heterogeneous catalyst for
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Fig. 6
One pot Suzuki borylation followed by Suzuki–Miyaura cross coupling.
borylation reaction. The ligands used (dppf, PCy3) are already known for their role in homogeneous catalysis for the same borylation reaction. By employing these ligands together with metal Pd NCs, new pathways in the catalysis eld can be opened. In this section we present the further investigations on borylation reactions catalysed by organic–inorganic hybrid Pd NCs, previously reported by our group.17 Pd@dppf and Pd@PCy3 have been chosen as catalysts due to their high activity in the borylation reaction of 4-bromoanisole with di isopropylaminoborane instead of the traditional bis(pinacolato) diboron or pinacolborane.34 For some substrates bearing no substitution (Table 2, entry 1) or electron donating groups (Me, OMe and NMe2, Table 2, entries 2–6), the reactions proceed cleanly and a simple ltration over a silica pad is enough to afford pure boronate. For the two catalyst types (Pd@PCy3 and Pd@dppf NCs) the yields are very similar except for the non-substituted arene ring (entry 1) where the Pd@PCy3 NCs produced a quantitative yield in boronate. These results are in agreement with the expected ones based on our previous work. In the case of polysubstituted substrates and other arene type substrates, moderate yields are obtained for both the systems. Note that in all these series, column chromatography was necessary because the reduction product (e.g. substitution of the bromide by a hydrogen) cannot be removed under vacuum (except for entries 12 and 13, Table 2) and in some cases, the reaction does not go to completion (entries 8, 12 and 13). The 2-OMe and 2-Me substitution (entries 12–13, Table 2) was found to be difficult to borylate due to steric hindrance of the reactive center. Compared with the acceptable yield obtained in homogeneous catalysis for the substrates bearing electron-withdrawing groups, the rst results, using organic–inorganic hybrid Pd NCs, obtained in the eld open an avenue for further investigation. Nonetheless, those NCs can be used to perform a one pot borylation–cross coupling sequence (Fig. 6),17,21 using the same palladium source for both reactions. In terms of time stability and catalytic efficiency of the hybrid Pd NCs, we performed borylation reaction with a solution aer a few months of storage in toluene. In the case of the Pd@dppf system the reaction yield decreased from 98% (fresh solution) to 50% (Table 2, entry 3), not surprising for a system containing the biggest NPs (Fig. 5c). Similar catalytic activity (yield 98%, Table 2, entry 3) for the Pd@PCy3, fresh and stored toluene dispersion was found, as expected for a thermodynamically stable system.
Conclusions In summary, a combination of highly active Pd NCs with surface property modulation (different ligands) leading to
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Paper organic–inorganic hybrid NC types, prepared via the continuous supercritical uid assisted coow route, seems to be promising catalysts in borylation reactions. Indeed, regardless of the meta or para substitution, bromides were converted almost quantitatively to the corresponding boronates, without the need for column chromatography. Sterically demanding aryl bromides were found to be more difficult to transform cleanly because the reduction was the favored reaction mechanism. Although the employed ligands in homogeneous catalysis give good results for the borylation reaction, even for substrates bearing electron withdrawing groups, in a colloidal system, hybrid type, the behaviour is different, conrming again the strong reactivity difference between homogeneous and semi-heterogeneous catalysts. Even so, the use of hybrid inorganic–organic NCs as catalysts opens the way for new synthetic pathways, enabling the possibility of catalyst surface modulation. This implies a faster practical alternative for adjusting and screening the reaction conditions.
Acknowledgements Acknowledgments are given to the R´ egion Aquitaine, GIS AMA and the ANR NANOCAUSYS no. 12-CDII-0010 for funding.
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